OBJECTIVES: Poly ADP-ribosylation of basic chromosomal proteins has been reported to occur both in vivo and in vitro. Functionally, this modification may present a mechanism for disrupting the electrostatic attraction between positively charged basic proteins and negatively charged chromosomal DNA. The reaction is thus likely to be involved in altering the degree of chromatin condensation during such processes as DNA and RNA synthesis, DNA repair or in preparation for mitosis. The objective of this project is to define in detail the changes in the poly ADP-ribosylation of nuclear proteins in actively growing mammary tumors and to quantitate the changes in this process when tumor regression is induced.